Occurrence of hexavalent chromium in ground water in the western Mojave Desert,California |
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| Institution: | 1. State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China;2. State Environmental Protection Key Laboratory of Simulation and Control of Groundwater Pollution, Chinese Research Academy of Environmental Sciences, Beijing 100012, China;3. College of Water Sciences, Beijing Normal University, Beijing 100875, China;1. Wise Laboratory of Environmental and Genetic Toxicology, University of Louisville, 500 S. Preston St, HSC55A Rm 1422, United States of America;2. Department of Pharmacology and Toxicology, University of Louisville, Louisville, KY, United States of America;3. Department of Pharmaceutical Sciences, University of New Mexico, Albuquerque, NM, United States of America;4. Pediatric Research Institute, Department of Pediatrics, University of Louisville, Louisville, KY, United States of America;5. School of Health Sciences, Purdue University, West Lafayette, IN, United States of America;6. Department of Environmental and Occupational Health, School of Public Health and Information Sciences University of Louisville, Louisville, KY, United States of America |
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| Abstract: | About 200 samples from selected public supply, domestic, and observation wells completed in alluvial aquifers underlying the western Mojave Desert were analyzed for total dissolved Cr and Cr(VI). Because Cr(VI) is difficult to preserve, samples were analyzed by 3 methods. Chromium(VI) was determined in the field using both a direct colorimetric method and EPA method 218.6, and samples were speciated in the field for later analysis in the laboratory using a cation-exchange method developed for the study described in this paper. Comparison of the direct colorimetric method and EPA method 218.6 with the new cation-exchange method yielded r2 values of 0.9991 and 0.9992, respectively. Total dissolved Cr concentrations ranged from less than the 0.1 μg/l detection limit to 60 μg/l, and almost all the Cr present was Cr(VI). Near recharge areas along the mountain front pH values were near neutral, dissolved O2 concentrations were near saturation, and Cr(VI) concentrations were less than the 0.1 μg/l detection limit. Chromium(VI) concentrations and pH values increased downgradient as long as dissolved O2 was present. However, low Cr(VI) concentrations were associated with low dissolved O2 concentrations near ground-water discharge areas along dry lakes. Chromium(VI) concentrations as high as 60 μg/l occurred in ground water from the Sheep Creek fan alluvial deposits weathered from mafic rock derived from the San Gabriel Mountains, and Cr(VI) concentrations as high as about 36 μg/l were present in ground water from alluvial deposits weathered from less mafic granitic, metamorphic, and volcanic rocks. Chromium(III) was the predominant form of Cr only in areas where dissolved O2 concentrations were less than 1 mg/l and was detected at a median concentration of 0.1 μg/l, owing to its low solubility in water of near-neutral pH. Depending on local hydrogeologic conditions and the distribution of dissolved O2, Cr(VI) concentrations may vary considerably with depth. Samples collected under pumping conditions from different depths within wells show that Cr(VI) concentrations can range from less than the 0.1 μg/l detection limit to 36 μg/l in a single well and that dissolved O2 concentrations likely control the concentration and redox speciation of Cr in ground water. |
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